At present, the development of superconducting electrical machines whose electrical circuits have the values of their resistances approaching zero is important. There exist materials which feature the superconductivity phenomenon at a temperature close to the absolute zero temperature. Cryogenic liquid such as liquid helium is therefore used to reduce the temperature of an electric winding of a superconducting electrical machine to a temperature at which the winding becomes superconducting.
There are a number of superconducting electrical machines which utilize different cooling circuits for cooling superconducting windings. Most of the specialists working on the given problem consider the superconducting electrical machines based on a standard cooling circuit the most favorable. In such a machine the inductor is a rotatable component and the armature is an immovable one. The inductor armature is made of a superconducting material and is supplied with d.c. excitation, while the armature winding is non-superconducting or is made of a pure or superconductor material.
The above-described machines have rather serious disadvantages as follows. The liquid helium used to cool the superconducting winding is subject to compression due to cetrifugal forces with the result that its temperature and pressure are increased. An increase in the temperature of the liquid helium also results from the inflow of heat to it along the design components of the rotor shaft and the conductors connecting the superconductor winding to the current-collecting device and to the power supply both located outside the machine.
An increase in the temperature of the liquid helium always results in an increase in the temperature of the superconducting winding, which in turn results in a reduced efficiency of the machine; in some cases, the superconducting state and good operating condition of the machine are also disturbed.
To counteract the increased temperature of the liquid helium caused by centrifugal compression, the following methods are used: the superconducting winding is supplied with overcooled helium; a portion of a refrigerator and, particularly, its throttling stage is disposed in the rotatable rotor; and the pressure of the gaseous helium above the liquid helium is reduced or the thermal conductivity of one of the elements of the machine rotor is employed to transmit the heat produced in the liquid helium distributed over the periphery of the rotor to the liquid helium in the vicinity of the axis of rotation of the rotor. To restrict ingress of heat along the design elements of the rotor shaft and the connectors, the employed methods are as follows: a material of low thermal conductivity is disposed between the heat-absorbing and heat-emitting portions of the rotor; the latter location is cooled additionally using an intermediate coolant; use is made of the constructions in which the elements generating heat or conducting it to the "low" zone are cooled along the overall length with the help of the gaseous helium outgoing from the superconducting winding.
Known in the art is a superconducting electrical machine (cf. U.S. Pat. No. 3,729,640 (1973)) having a rotor with a superconducting winding supplied with d.c. excitation. There is a heat-transfer member within the rotor which is designed to transfer heat radially from the liquid helium distributed over the external wall of the rotor to the liquid helium passing through an axial feed passage to the superconducting winding. The heat-transfer member is a thick-wall disc located at the end face of the superconducting winding and brought into thermal contact with the liquid helium which surrounds the axial space of the rotor and is distributed over its periphery. However, this embodiment offers poor effectiveness since the area of the axial feed passage for the liquid helium is small. Moreover, the machine has a complex construction in this case for the following reasons: first, the heat-transfer member must provide for efficient removal of heat and must have an extended surface for this purpose; secondly, that member must be compact and robust to resist the action of centrifugal forces applied to it. It is known, however, that materials possessing good thermal conductivity such as copper and aluminum feature low mechanical strength and must be therefore reinforced by a material having higher mechanical strength and not affected appreciably by intense magnetic fields. Note that the time lag of the heat-transfer member does not allow it to resist an increase in the temperature of the superconducting winding during unstable operation of the machine occurring in the start mode and during transient conditions. Finally, there exist great inflows of heat along the shaft and conductors to the superconducting winding.
Known in the art is an electrical machine with cryogenic cooling (cf. the USSR Inventor's Certificates Nos. 476,638 and 262,240), which comprises a hollow rotor with a superconducting field winding connected to supply buses, a cooling system incorporating a feed passage for delivering a coolant (such as helium) to the superconducting field winding located along the axis of the rotor shaft, a means for cooling the end parts of the rotor shaft, and passages for withdrawing the coolant from the superconducting winding.
To cool the rotor shaft end parts, use is made of heat-insulating stoppers mounted within a cylindrical portion of the hollow rotor at the locations where that cylindrical portion connects with the rotor shaft. The mating surfaces of the heat-insulating stoppers and of the cylindrical portion are provided with ribs arranged helically.
To reduce the pressure of the gaseous helium and the resistance of the cooling ducts, the gaseous helium is ejected into a space between the stator and the rotor and is then passed to a refrigerator.
The heat conducted into the rotor cavity tends to evaporate the liquid helium and the vapor so produced passes into the spaces between the interior of the cylindrical portion of the rotor and the exterior of the heat-insulating stoppers. When passing through the spaces between the ribs to the passages for withdrawing the coolant, the heat-absorbing gaseous helium tends to absorb the heat conducted from the surrounding medium along the end faces of the rotor shaft.
In the known machine it is difficult to restrict the inflows of heat along the rotor shaft since a great amount of heat is transferred into the rotor cavity along the attachment points between the shaft end parts and the rotor cylindrical portion, which cannot be protected from outward ingress of heat; on the other hand, no protection from the inflows of heat along the conductors is provided.
The means for cooling the rotor shaft end parts, namely, the heat-insulating stoppers, results in sophisticated construction of the machine and increased dimensions thereof. With this construction, it is difficult, however, to restrict the inflow of heat along the shaft and conductors to the superconducting winding and, therefore, to the liquid helium. The flow rate of the coolant is also difficult to control at both end parts of the rotor shaft since the coolant, after passing through the cooling ducts in the stoppers, is ejected into a space between the rotor and stator and the machine has no coolant collecting chambers independent of the stator.
There is another electrical machine with cryogenic cooling (cf. the USSR Inventor's Certificate No. 484,606 dated May 5, 1973) which utilizes an additional coolant to reduce the inflow of heat along the shaft. Mounted on each end part of the rotor shaft, on the sides adjacent the end face walls of the cylindrical portion of the hollow rotor, is a chamber for the coolant. This chamber comprises coaxial discs (or cylinders) having a common wall, tubings for introducing the coolant being passed through the discs and rigidly mounted relative to the chamber, and mating discs attached to the stator and surrounding the firstmentioned discs. There are seals in a space between the movable and immovable discs.
The described embodiment requires a sophisticated construction of the machine which possesses a poor reliability in this case since reliable vacuum-tight rotatable seals operated at cryogenic temperatures are difficult to create. In addition, a considerable increase in the length of the rotor shaft is caused due to the fact that the chambers are installed on the shaft end parts. Moreover, no effective protection from the inflow of heat along the shaft is provided since the portions of the shaft end parts running from the chambers to the end face walls of the cylindrical portion of the rotor have no thermal protection at all and an increase in their size would affect the mechanical strength of the rotor. As a result, there results a considerable inflow of heat to the superconducting winding and the liquid helium along the shaft from the locations where the chambers are installed which are maintained at a temperature in the range from 20.degree. to 80.degree. K. to the superconducting winding. When another liquid coolant with a lower temperature is used, poor thermodynamic characteristics are obtained. In the described machine no protection from the inflow of heat to the superconducting winding along the conductors is provided.
Known in the art is an electrical machine with cryogenic cooling (cf. British Pat. No. 1,541,550) having a hollow rotor filled with a liquid helium. The rotor comprises a superconducting winding with at least two supply leads, one of the supply leads being connected to a plus sign current-collecting means and being disposed in at least one cooling duct for cooling one and part of the hollow shaft of the rotor. The cooling ducts for cooling both end parts of the hollow shaft of the rotor are connected, via respective exhausting means for withdrawing the coolant from the rotor, with respective coolant collecting chambers. The other supply lead is coupled to a minus sign current-collecting means. The hollow shaft being provided with a passage for feeding the coolant to the superconducting winding, said passage being connected with an inlet means for introducing the coolant into the rotor, said inlet means being disposed in one end part of the hollow shaft and being supported in bearings each having a housing with a seal.
In the known machine the end parts of the hollow shaft of the rotor are provided with multiturn helical ducts accomodating the supply leads, which tends to reduce the inflow of heat to the superconducting winding disposed within the hollow rotor. The supply leads used to connect the superconducting winding to the plus and minus sign current-collecting means are disposed in the ducts belonging to one end part of the hollow shaft of the rotor, which comprises the inlet means. The current-collecting means comprise rotatable rings of electric conductivity material and carbon brushes arranged in fixed relation to the rings and cooled due to heat exchange with the surrounding medium. The cooling ducts of the end parts of the hollow shaft of the rotor are connected, via the exhausting means implemented in the form of radial passages in the shaft body, with the coolant collecting chambers having their cylindrical casings surrounding the end parts of the hollow shaft, said casings having their end face walls provided with gas-tight seals. To provide for thermal insulation between the superconducting winding and the environment, the space between the rotor and stator is continuously maintained under a vacuum. For this purpose, the endshields of the stator casing mount, in addition to the bearings supporting the rotor, rotatable vacuum-tight seals.
Since the supply leads connected to the current-collecting means of plus and minus sign are disposed in common in the cooling ducts of one of the end parts of the hollow shaft of the rotor, these supply leads are difficult to insulate thermally and their cooling surface cannot be made extended; as a result, heat is removed from them with difficulty. The cooling of the current-collecting means by effecting heat exchange with the environment gives insufficient results and a great amount of heat generated therein is therefore transferred to the shaft and supply leads along which the heat is conducted directly to the superconducting wiring and the liquid helium available in the hollow rotor. In addition, a joint arrangement of the supply leads connected to the current-collecting means of plus and minus sign does not allow for an optimum operating condition of these leads since they conduct heat to the superconducting winding when the machine is started or when transient conditions take place. The exhausting means for withdrawing the coolant from the ducts in the end parts of the hollow shaft of the rotor, made in the form of radial passages in these end parts, possess a low effectiveness since the radial passages have a higher hydraulic resistance for the coolant at their inlets and outlets. It is also difficult to control the flow rate of the coolant through the cooling ducts of the end parts of the hollow shaft of the rotor, which is necessary during current surges caused by unstable operation of the machine, since there is no additional bypass passage for the coolant which would be independent of the cooling ducts for cooling the end parts of the hollow shaft of the rotor.
Due to the presence of a large number of rotatable seals at the end parts of the rotor shaft, the known machine features a complex construction and poor reliability. The gaseous coolant delivered to the coolant collecting chambers from the cooling ducts of the end parts of the hollow shaft of the rotor is contaminated in these chambers since there is foreign matter due to wear of the rotatable gas-tight seals and due to an evaporation of the liquid (oil) responsible for hermetic condition of these seals. As a result, one cannot attain an adequate cooling of the superconducting winding by reducing the pressure in the rotor cavity and by restricting the inflow of heat to this winding; moreover, the individual elements of the rotor and the machine as a whole cannot be given high reliability properties.